1. Field of the Invention
The invention relates to a transparent cover layer for windshields or other transparent glass or plastic substrates of soft elastic polyurethanes which is made from a reaction mixture, the isocyanate component of which contains at least one essentially trifunctional aliphatic polyisocyanate based on 1,6 hexamethylene diisocyanate with a biuret or isocyanurate structure having a content of NCO groups of from 12.6 to 28 percent by weight and an average molecular weight of about 450 to about 1000 g/mol, and the polyol component of which contains a difunctional sulfonated polyether polyol or a difunctional polyether polyol having 0.5 to 13.2 percent by weight of OH groups and an average molecular weight of about 250 to 6500 g/mol and a trifunctional polyol based on trimethylol propane and propylene oxide with a 5.1 to 12.8 percent by weight content of OH groups and an average molecular weight of about 400 to about 1000 g/mol.
2. Background of the Prior Art
Transparent soft elastic cover layers of this type are the object of the German Patent Application P 36 05 765.7. On the one hand they have the characteristics that, when subjected to common types of loads, which with harder plastics lead to superficial deformations and scratch-like impressions, they experience only an elastic deformation which disappears after a short time, and on the other hand they have the advantage that the formation of bothersome condensate fog is reduced or eliminated. The reduction or elimination of disruptive condensate fog is accomplished by means of a water-spreading effect which is achieved through the use of difunctional sulfonated or sulfonate-group-containing polyether polyols in the reaction mixture for the manufacture of the cover layers.
Soft elastic cover layers of this type can either be manufactured directly onto the underlayer to be coated by applying the appropriate surface with the reaction mixture, or it can be applied as a premanufactured foil to the underlayer to be coated. In the latter case the foils are produced through application of the reaction mixture onto a casting base, whereby the foils are pulled from the casting base after the reaction is complete, i.e. after the reaction mixture has hardened.
It has been shown that the fog-reducing characteristics of the prefinished foils can be different on the two surfaces. If, for example, the casting base is made of metal or a glass plate, then the anti-fogging effect of a foil produced on such a casting base is substantially less on the upper surface, i.e. on the surface that was exposed to the ambient atmosphere, than on the surface that was in contact with the casting base.
The object of the invention is to improve further the fog-reducing effect of the cover layers described above, particularly on the surface that is in contact with the surrounding atmosphere during the forming of the cover layers from the reaction mixture.